Bipolar batteries have been used to improve battery energy storage capacity on a weight and volume basis, to reduce packing weight and volume, to provide stable battery performance and low internal resistance.
A bipolar battery construction comprises an electrically conductive bipolar layer, a so called biplate, that serves as electrical interconnection between adjacent cells in the battery as well as a partition between the cells. In order for the bipolar construction to be successfully utilized, the biplate must be sufficiently conductive to transmit current from cell to cell, chemically stable in the cell's environment, capable of making and maintaining good contact to the electrodes and capable of being electrically insulated and sealable around the boundaries of the cell so as to contain electrolyte in the cell.
These requirements are more difficult to achieve in rechargeable batteries due to the charging potential that can accelerate corrosion of the biplate and in alkaline batteries due to the creep nature of electrolyte. Achieving the proper combination of these characteristics has proven very difficult. For maintenance-free operation it is desirable to operate rechargeable batteries in a sealed configuration. However, sealed bipolar designs typically utilize flat electrodes and stacked-cell constructions that are structurally poor for containment of gases present and generated during cell operation. In a sealed construction, gases generated during charging then need to be chemically recombined within the cell for stable operation. The pressure containment requirement creates additional challenges on designing a stable bipolar configuration.
In a bipolar battery there is, as mentioned above, a need for electrodes that should be in good contact with the biplate. Presently, the negative and positive electrode being in contact with the biplate are manufactured separately and treated in such a way to improve contact with the biplate when attached to it. Such treatments are disclosed in U.S. Pat. No. 5,611,823, by Klein, where the electrodes are manufactured from electrochemically active material prepared by coating non- to low-conductive particles of a metal hydroxide or metal oxide powder with nickel using a electroless nickel coating process.
Other treatments to manufacture individual robust, positive and negative electrodes is to utilize a current collecting grid for the dual purpose of supporting the electrochemically active materials, and conducting the electric current from the electrode via the conductive grid.
Pressed powder has previously been used when manufacturing separate electrodes. An article, Ni-MH Battery Electrodes Made by a Dry Powder Process was published in the Journal of the Electrochemical Society, Vol. 142, Number 12, December 1995 (“the ECS article”), and is hereby incorporated by reference. Relevant disclosure from the ECS article is bodily incorporated below in paragraphs [0008-20] and in FIGS. 8-27.
The high specific capacity of the Ni-MH battery and the rapidly increasing demand for secondary batteries have led to a rapid increase in the production of Ni-MH batteries. In Japan, the output has doubled each year. For 1994, it reached 206 million cells. In the future, the Ni-MH battery may also be an alternative for electric and hybrid vehicles. For a continuing growth of the Ni-MH market share it is, however, important to reduce the cost of the batteries.
The relative simplicity of the electrode reactions in the Ni-MH battery can help to develop attractive and low-cost batteries. Both electrode reactions according to Equations 1-3, infra, are solid-state intercalation reaction of hydrogen. When the cell is charged, hydrogen is transferred from the Ni(OH)2, which forms NiOOH, to make a water molecule at the electrode surface with an OH− ion from the electrolyte. At the metal hydride (MH) electrode another water molecule is decomposed and a hydrogen atom is intercalated and stored in the MH electrode. When the cell is discharged, the procedure is reversed. In contrast to the Ni—Cd and lead-acid batteries, no water is produced or consumed in the overall cell reactions, and the amount of the electrolyte is thus constant and it participates only as an ionic charge carrier. The metal hydride is further a good electronic conductor both in its charged and discharged state. The main problem is the positive electrode where Ni(OH)2 is an insulator in the discharge state of the electrode. A substantial part of the work presented by the authors of the ECS article, Ye et al. (collectively “the researchers”) was, therefore, aimed at improving the conductivity properties of this electrode. Both electrodes were, however, made by cold-pressing dry electrode powder onto an endless metal net, which was a method developed by Svenska Ackumulator AB Jungner in Sweden, in the beginning of 1970s, for the mass production of low cost Ni—Cd consumer batteries. In the present project the same roller pressing equipment was used to produce electrodes for the researchers' Ni-MH test cells. In the beginning of the 1970s, as much as 46 weight percent (w/o) carbonyl nickel powder had to be added to the positive electrode as a binder as well as to enhance the conductivity, in order to obtain good utilization of the nickel hydroxide and an acceptable rate capability for the produced batteries, which increased cost and weight. But now, due to improved nickel hydroxide and carbonyl nickel qualities and the positive influence of Co addition on the performance of the nickel hydroxide, the researchers thought it would be possible to produce commercial batteries by this method, with a performance that can be compared to that of batteries made by Ni foam/felt methods. Furthermore, this dry powder method is more environmentally acceptable than Ni-sinter technologies because it includes no wet chemical steps and reduces spillage and disposal of by-products.Positive:Ni(OH)2+OH−NiOOH+H2O+e−(Eo≈+0.490 V)  [1]Negative:M+H2O+e−MH+OH−(Eo≈−0.828 V)  [2]Total:Ni(OH)2+MNiOOH+MH(Eo≈1.318 V)  [3]
An electrode manufacturing method, originally developed for the production of low cost Ni—Cd cell, has been modified to produce electrodes for Ni-MH cell (sub-C size) in the range 1.5-2.5 Ah. The earlier process reported in U.S. Pat. No. 3,640,772 was dependent on a special treatment of the active material with a polymer dissolved in a solvent. In this treatment, a slurry was made with the active materials and the wet additives after which the solvent was evaporated and the polymer-treated active material formed a solid which was ground into a suitable particle size. In the process described in this paper there is no need for this treatment of the active material. The high-density nickel hydroxide powder and the mischmetal hydrogen storage alloy powder, mixed with certain amount of fine nickel and cobalt metal powder, are directly fed to both sides of a continuous net of woven pure nickel wire at ambient temperature and compacted in an air atmosphere to make electrodes in the form of “endless” bands. No sintering step is needed. The roller pressing equipment is schematically shown in FIG. 8. Finally the bands are cut into suitable electrode lengths. The electrical leads are connected by spot welding and the electrodes are directly coiled together with a separator and inserted into battery cans with a sealing lid for the subsequent evaluation.
The battery characteristics were examined with a PC-controlled multichannel battery charge/discharge system which allows each charge/discharge cycle to be recorded with a 10 mV measuring step and stored on the hard disk of a computer. Time and voltage could both be controlled, and the current could be varied from 10 mA to 20 A. The non-ambient temperature performance of some batteries was measured at the Electrolux Battery Center in Stockholm, Sweden.
The active materials used in this test, i.e., hydrogen storage alloy MmNi3.6Co0.7Al0.35Mn0.35 (Mm; 50-60 w/o La, 30-40 w/o Ce, and about 10 w/o Nd and Pr) and nickel hydroxide (containing 2-4 w/o Co and 3-6 w/o Zn), were generously supplied by Gesellschaft fur Elektrometallurgi in Nürnberg, Germany, and Tanaka Chemical Corporation in Fukui, Japan, respectively. Carbonyl nickel powder (Inco 255, 210) and cobalt powder (MHO extra fine) were used as binders and as conducting additives in both electrodes. The MH electrode composition was fixed at 85 w/o AB5 alloy, 10 w/o Ni, and 5 w/o Co. The carbonyl nickel content in the positive electrode was varied from 46 to 15 w/o, and the effect of cobalt addition was investigated up to 7 w/o. The effects of two kinds of electrolyte were tested, 6 M KOH+1 M LiOH and 10 M KOH which was used in Ni—Cd cells in 1970s. A polyamide nonwoven cloth with thickness 100 μm (FT2119) in two layers was used as separator.
During the work for the ECS article, the researchers came to the conclusion that the positive Ni(OH)2 electrode is in many aspects the weak partner in the Ni-MH battery couple. And because the battery must be Ni-electrode limited in order to enable proper recombination reactions, the researchers put much emphasis on the optimization of this electrode. To improve the electronic conductivity, carbonyl nickel was added to the Ni(OH)2 so that the fine nickel particles form a dendritic structure when pressed together with the Ni(OH)2. Cobalt was also added to improve charge efficiency and utilization. FIG. 9 shows the importance of Co addition and the influence of two different nickel powder qualities (Inco 210 and 255) on the Ni(OH)2 utilization. The utilization of the NiOOHNi(OH)2 reaction refers to a theoretical value of 289 mAh/g of Ni(OH)2 after subtracting the weight of the coprecipitated Zn(OH)2. This Zn(OH)2 addition (amounting to 3.5 w/o) is made by the producer of our starting material in order to suppress a formation of γ-NiOOH which causes electrode swelling and a dry out of the separator upon cycling. The Co additive is further especially important when the amount of conducting Ni powder is reduced. In FIG. 9, the researchers also found that finer grain size Ni (Inco 210) is more effective for increasing the utilization. With this type of powder-pressed electrode the researchers were able to reach a specific capacity of 550 mAh/cm3 at a 0.2 C discharge rate. This is close to what can be obtained with Ni-foam/felt technology. FIG. 10 shows the charge and discharge voltage of an initially optimized battery at a 0.1 C rate. It can be seen that the capacity of the battery is close to 2.4 Ah (i.e., 180 Wh/dm3, 56 Wh/kg) and almost the same as the present commercial cells. This fairly good value can be understood after seeing FIG. 11, where the volume ratio of active material in the three electrode types is compared. The amount of active material in the powder pressed electrode is only about 5 volume percent (v/o) lower than in the foam/felt electrodes leading to comparable total capacities for cells using these two electrode types.
The loss of cycle life is usually caused by a loss of the electrolyte, resulting in a dry out of the cells. A primary cause for electrolyte dry out, which is common for both Ni-MH and Ni—Cd cells, is the incorporation of water molecules in the nickel hydroxide, leading to a swelling of the electrode and a removal of the electrolyte from the separator. Also the corrosion of the metal hydride consumes the electrolyte as well as poor recombination reactions which cause excessive cell pressures and a venting of the cells through the safety valve. In the final stage of the life-span of a hydride battery, both of the latter processes usually occur; this is the reason for the fairly rapid decay of the hydride battery capacity when the battery nears the end of its life expectancy. A typical hydride battery initially has a fairly stable capacity over several hundreds of cycles but eventually reaches a sudden reduction, compared to the Ni—Cd battery where the capacity reduction sets in earlier but is more gradual.
To emulate a suitable starved electrolyte, the researchers varied the amount of electrolyte added and recorded its influence on the internal pressure and internal resistance, as seen in FIGS. 12 and 13. The pressure was measured by removing the safety valve and connecting the cell to an external pressure gauge. With decreasing amounts of electrolyte, the internal pressure decreased as shown in FIG. 12, while the internal resistance increased as shown in FIG. 13, indicating an optimized amount of electrolyte of about 6 g. To prevent the pressure rise during overcharge, an excess of the metal hydride to the nickel electrode (CMH/CNi=1.3) is also needed. This facilitates the recombination reaction of oxygen as well as suppressing a side reaction evolving hydrogen gas at the negative electrode, as can be seen in FIG. 14.
At the beginning, the cycle life was tested by using the IEC standard for Ni—Cd batteries, which means that the cells were charged at the 0.25 C rate for 3 h and 10 min and discharged at the 0.25 C rate for 2 h and 20 min as shown in FIG. 15 (A 1 C rate would correspond to a current that would charge the cell to its nominal capacity during 1 h). The researchers thought, however, that some of the capacity drop is due to an extensive overcharge caused in this IEC standard. The very good charge efficiency of the hydride batteries makes the rather extensive overcharging used for Ni—Cd cells unnecessary. The charge and discharge efficiency for the cells is shown in FIG. 16. Up to about 2.16 Ah of charge input or about 90% of the full 2.4 Ah charging capacity, the charge efficiency is very close to 100%. This is a very important advantage when making large cells, as the amount of dissipated heat that has to be removed can be kept very low, if the cells are not overcharged. This also means that the IEC standard for cycling Ni—Cd cells is not suitable, as it will overcharge the batteries for almost an hour during each cycle. By increasing the discharge time to 2 h and 50 min we could notably reduce the loss of cyclic capacity by removing charge from the cells and thus reducing the amount of overcharge in the subsequent cycle as shown in FIG. 17.
The cobalt addition to the positive electrode, especially at low carbonyl nickel content, was found to be beneficial for the cycle life. As seen in FIG. 15, a 35 w/o addition of carbonyl nickel gives a fairly high utilization, but a very poor cycle life. If 5 w/o of the nickel is replaced by cobalt, the utilization is further increased and the cycle life is dramatically improved. The formation of conducting CoOOH became more important for increasing the charging and discharging efficiencies of the Ni(OH), when the carbonyl nickel content was decreased. Furthermore, the Co addition to the positive electrode influences not only this electrode directly, but also indirectly the negative hydride electrode. FIG. 18 compares the remaining capacity in two sealed cells that have been cycled 350 times at a 0.5 C rate, charging for 2 h and a 0.5 C rate, discharging for 1.6 h. After cycling, the cells were opened and refilled with electrolyte and the discharge curves of both electrodes were measured against an Hg/HgO reference electrode. Initially both cell No. 319 and No. 338 had the same negative overcapacity. After the cycling the negative overcapacity of the cell No. 319 using a Co-free nickel electrode has been lost. In the end, the No. 319 cell became hydride electrode limited. This is the reason that batteries using cobalt-free nickel electrodes quickly lost their capacity as seen in FIGS. 15 and 19. The researchers attributed this corrosion to a poorer charge efficiency of the Co-free Ni electrode, as seen in FIG. 16, causing an evolution of oxygen, which has a detrimental effect on the hydride electrode when it is not fully charged. This is also indicated by the different activation processes for the two kinds of batteries using Co-added and Co-free nickel electrodes. In FIG. 20, the first charge/discharge curves of two different batteries are shown. The form of the charge curves are quite different. The first step of about 1 V in curve b (Co-added nickel electrode) is attributed to the reaction Co(OH)2CoOOH and MMH. At this voltage, almost no oxygen evolution is expected and the hydrogen storage alloy is prevented from oxidization. When the cell is more fully charged, the evolved oxygen can be recombined with the hydrogen in the metal hydride, thus preventing the negative electrode from corroding. But when no Co has been added to the positive electrode, the charge voltage, even at the beginning, is higher (1.4 V) as shown in curve a. At this voltage, oxygen can be evolved at the Ni-electrode and react with the hydrogen storage alloy in the MH-electrode.
The temperature performance was measured at the Electrolux RI Battery Centre in Stockholm, Sweden. FIGS. 21-23 show the discharge curves of the batteries at different temperatures. The cells were charged at room temperature at the 0.2 C rate and rested for at least 2 h, during which time the discharge temperature was established. Cell No. 961 (FIG. 15) had a 10 M KOH electrolyte. Cell No. 359 (FIG. 21) and No. 966 (FIG. 23) had a (6 M KOH+1 M LiOH) electrolyte; cell No. 359 had been through a test with 250 full charge and discharge cycles at a 0.5 C rate before this temperature experiment was done. Comparing the discharge voltage in FIGS. 21 and 23, it is interesting to note that no significant increase in the internal resistance could be observed as a result of these initial 250 cycles. The temperature dependence of the discharge capacity as shown in FIG. 24 was found to be rather flat between −18 and 40° C., except for battery No. 961 with a 10 M KOH electrolyte, where the increased internal resistance at low temperature significantly reduced the capacity at −18° C.
FIG. 25 shows the charge retention at 20° C. The charge retention of the battery was measured as follows. The battery was charged at a 0.1 C rate for 15 h; the charged battery was kept at room temperature for the number of days plotted in the FIG. 25, after which the battery was discharged at a 0.2 C rate to 1 V. The self-discharge rate of the batteries was not large, and we think it can be further improved by replacing the polyamide separator with sulfonated-polypropylene separator.
Two electrolytes, 6 M KOH+1 M LiOH and 10 M KOH which was used in Ni—Cd cells in the 1970s were tested. The 10 M KOH gave a little higher initial capacity as shown in FIG. 26, but also a lower discharge voltage (FIGS. 22 and 26) and a higher internal resistance as shown in FIG. 27. The low temperature performance of the battery with a 10 M KOH electrolyte was very poor as shown in FIGS. 22 and 24.